JP2003041346A - Duplex stainless steel for radioactive material storage container and method for manufacturing radioactive material storage container - Google Patents

Abstract

(57) [Summary] [Problem] To provide a duplex stainless steel excellent in corrosion resistance, toughness, electron beam weldability and usable as a material of a radioactive material storage container, and a method of manufacturing a radioactive material storage container using the duplex stainless steel. . SOLUTION: In mass%, C: 0.005 to 0.030%, Si: 0.05
~ 0.75%, Mn: 0.20 ~ 1.00%, Ni: 5.5 ~ 7.5%, Cr: 2
4.0-26.0%, Mo: 2.5-3.5%, Cu: 0.2-0.8%, W:
0.1 to 0.5%, N: 0.20 to 0.30%, Al: 0.010 to 0.050%,
Ca: 0 to 0.0050%, B: 0 to 0.0030%, balance Fe and impurities, P in impurities is 0.035% or less, S is 0.005%
% Or less, Ti is 0.05% or less, Nb is 0.1% or less, V is 0.5% or less. Duplex stainless steel for a radioactive material storage container, and welding energy of this duplex stainless steel is 7.7 to 18.0k.
A method for producing a radioactive substance storage container, comprising assembling by J / cm ² electron beam welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless steel for a radioactive substance storage container and a method for manufacturing a radioactive substance storage container by assembling the duplex stainless steel by electron beam welding.

[0002]

2. Description of the Related Art Spent nuclear fuel has conventionally been stored in water and is supposed to be reprocessed. Therefore, a short storage period of several years was assumed in nuclear power plants and reprocessing facilities. . However, in recent years, as the storage amount has increased, and the limit of the processing amount has been predicted by reprocessing, storage of spent nuclear fuel for a long period of time has been considered. In addition, the amount of high-activity waste that should be discarded is increasing, and similar long-term storage is being considered. In that case, for spent nuclear fuel and waste with high radioactivity levels (hereinafter, both are simply referred to as radioactive substances), dry storage, which is superior in terms of cost and management, is desired instead of conventional storage in water. It is rare.

The storage of radioactive materials using closed storage containers is generally done in an environment where the factory leaves the lid and forms the main part of the closed storage container, after which sufficient measures are taken to shield the radioactivity. It is carried out by charging a radioactive substance and then sealing the radioactive substance by welding the lid portion so as to form a closed structure.

The closed storage container is in the shape of a cylinder having a diameter of about 2 m and a height of about 5 m, and an internal structure is built in the container and charged with a radioactive substance. Usually, radioactive material is charged by remote control. For example, in order to prevent damage to fuel rods, a closed storage container is required to have extremely high dimensional accuracy as a structure.

Traditionally, TIG welding has been employed in the manufacture of closed storage vessels in factories. However, in order to form a highly accurate closed storage container by TIG welding, it is necessary to reduce the welding speed, which is disadvantageous in terms of productivity. Moreover, there is a limit in obtaining high dimensional accuracy. Therefore, in order to solve such a problem, electron beam welding (Electric Beam Weldin
g) has been required to apply.

On the other hand, the closed storage container is often placed near the coast, and in order to prevent pitting corrosion and the like caused by flying sea salt particles, it is necessary to consider the corrosion resistance of the material used. That is, as a material for a radioactive substance storage container, not only mechanical properties such as strength and toughness but also excellent corrosion resistance are required. In particular, with respect to corrosion resistance, unlike temporary storage in water, it is required that corrosion problems do not occur even when used for several decades.

Japanese Patent Laid-Open No. 2001-141878 discloses that
A corrosion prevention method for preventing crevice corrosion and stress corrosion cracking of a reactor shielding material and a container for transporting and storing nuclear fuel is disclosed. In this invention, in order to secure the corrosion resistance of the container made of B-added austenitic stainless steel which has the effect of absorbing thermal neutrons, the Cl ion concentration and temperature of the operating environment are specified. However, it is difficult to always keep the storage container in such a usage environment,
Not realistic. Rather, it is more realistic to adjust the material composition of the storage container according to the expected usage environment rather than adjusting the usage environment.

As a material excellent in corrosion resistance, for example, 2 as described in JP-A No. 11-80901 is used.
Examples include duplex stainless steel. However, the corrosion resistance described in this publication is the corrosion resistance of the steel itself, that is, the base metal, and the weld metal portion and HAZ formed when welding is performed.
The corrosion resistance in (welding heat affected zone) is unknown. Therefore, duplex stainless steel as disclosed in JP-A No. 11-80901 is not always suitable as a material for a closed storage container for radioactive substances.

In particular, when electron beam welding is used when producing a closed container for radioactive materials, it is essential to weld in a vacuum, so that N contained in the steel is released in the melted portion, resulting in welding defects. However, there is a problem that the corrosion resistance is deteriorated. In addition to this, unlike electron beam welding, electron beam welding reduces the toughness of the welded part.
There is also a problem to be solved regarding the mechanical properties of the material of the storage container.

[0010]

The object of the present invention is that it can be used as a material for a radioactive substance storage container and has excellent electron beam weldability with good mechanical properties such as corrosion resistance and toughness.
Providing duplex stainless steel. Another object of the present invention is to provide a method of manufacturing a radioactive substance storage container by electron beam welding using the duplex stainless steel as a raw material.

[0011]

DISCLOSURE OF THE INVENTION The inventors of the present invention basically assume that a radioactive substance storage container must have electron beam welding, and therefore the radioactive substance storage container must basically have corrosion resistance and mechanical properties. In addition to characteristics such as dynamic strength,
In order to obtain a steel material to which electron beam welding can be applied, attention was paid to the material composition.

In the electron beam welding, unlike the ordinary TIG welding or the like, the base material is directly melted and joined without using a filler, so that the composition of the base material directly affects the weldability. In particular, in electron beam welding, N is released into a vacuum, so it is necessary to design the material so as to secure N in the material even after welding. Therefore, the material composition was examined based on duplex stainless steel having excellent corrosion resistance and mechanical strength.

In order to secure N in the material even after welding, considering the amount of released N, a large amount of N may be contained in the duplex stainless steel in advance. But such high N
Electron beam welding of the above steel reduces the toughness of the weld metal. When the cause of this decrease in toughness was investigated, high N
It was found that the cause is the generation of inclusions due to oxidization, and in particular, it has been found that the toughness is reduced by the fact that Ti mixed as an impurity forms N and TiN.

Further, in order to prevent the occurrence of welding defects, welding conditions suitable for the material composition are required. Therefore, the welding conditions were also examined.

The present invention has been completed based on the above-described examination results, and the gist thereof is as follows in (1) 2
It is in the manufacturing method of duplex stainless steel and the used radioactive material storage container of (2).

(1) C: 0.005 to 0.030% by mass%, S
i: 0.05 to 0.75%, Mn: 0.20 to 1.00%, Ni: 5.5 to 7.5
%, Cr: 24.0 to 26.0%, Mo: 2.5 to 3.5%, Cu: 0.2 to 0.8
%, W: 0.1 to 0.5%, N: 0.20 to 0.30%, Al: 0.010 to
0.050%, Ca: 0 to 0.0050%, B: 0 to 0.0030%, balance F
consisting of e and impurities, P in the impurities is 0.035% or less,
S is 0.005% or less, Ti is 0.05% or less, Nb is 0.1% or less, V
Is less than 0.5% and is a duplex stainless steel for used radioactive material storage containers with excellent electron beam weldability.

(2) Manufacturing of a used radioactive substance storage container characterized by assembling the duplex stainless steel of (1) above by electron beam welding with a welding energy per unit area of 7.7 to 18.0 kJ / cm ^2. Method.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The invention (1) of the duplex stainless steel (hereinafter referred to as the first invention) is an invention of the duplex stainless steel excellent in electron beam weldability. Here, "excellent in electron beam weldability" means that even if welding is performed by electron beam welding, no weld defect occurs in the welded portion and the welded structure can be formed with high dimensional accuracy.
The duplex stainless steel of the first invention is required to have a material composition within the range shown below. In addition, the content of each of the material compositions described below is represented by mass%.

C: 0.005 to 0.030% C is an austenite stabilizing element and also contributes to the improvement of strength. In order to obtain these effects, the content of 0.005% or more is required. However, if the content exceeds 0.030%, the amount of carbides generated in the HAZ increases, leading to deterioration of corrosion resistance. Therefore, the C content needs to be suppressed to 0.030% or less.

Si: 0.05 to 0.75% Si is used as a deoxidizer for steel, and it is necessary to contain Si in an amount of 0.05% or more. However, if the Si content exceeds 0.75%, the formation of the σ phase is promoted, and the corrosion resistance and toughness deteriorate. Therefore, the proper content of Si is 0.05 to 0.75%.

Mn: 0.20 to 1.00% Mn functions not only as a deoxidizer but also as an austenite forming element, so 0.20% or more is contained. Further, Mn has the effect of fixing S to improve hot workability, but when the Mn content exceeds 1.00%, the formed MnS becomes the starting point of pitting corrosion. Therefore, the Mn content is 0.20-1.00
%, Preferably 0.20 to 0.80%.

Ni: 5.5-7.5% Ni is an essential component of duplex stainless steel and has the effect of stabilizing the austenite phase. In order to exert its effect, the Ni content is 5.5 to 7.5%. If it is less than 5.5%, the ferritic phase increases and the properties as a duplex stainless steel deteriorate. On the other hand, if it exceeds 7.5%, the austenite phase increases too much, and the generation of the σ phase is promoted.

Cr: 24.0 to 26.0% Since Cr is an element that contributes to the improvement of the corrosion resistance of duplex stainless steel, it is contained at 24.0% or more. In order to secure even better corrosion resistance, it is preferable to contain 25.0% or more.
However, when the Cr content exceeds 26.0%, the generation of the σ phase is promoted and the hot workability deteriorates. Therefore, the Cr content is
24.0 to 26.0%. 25.0-26.0% is preferable.

Mo: 2.5 to 3.5% Like Cr, Mo is an element that contributes to the improvement of corrosion resistance. Therefore, 2.5% or more is contained. In order to secure excellent corrosion resistance, it is preferable to contain 3.1% or more. But Mo
If the content exceeds 3.5%, the generation of the σ phase is promoted and the hot workability deteriorates. Therefore, the Mo content is 2.5 to 3.5.
%, Preferably 3.1 to 3.5%.

Cu: 0.2 to 0.8% Cu, like Cr and Mo, is an element that contributes to the improvement of corrosion resistance. Therefore, 0.2% or more is contained. However, if the Cu content exceeds 0.8%, the hot workability deteriorates. Therefore, the appropriate Cu content is 0.2 to 0.8%.

W: 0.1 to 0.5% W, like Cr, Mo and Cu, is an element contributing to the improvement of corrosion resistance. Therefore, 0.1% or more is contained. However, when the W content exceeds 0.5%, the generation of the σ phase is promoted and the hot workability deteriorates. Therefore, the W content is 0.1 to 0.5%.

N: 0.20 to 0.30% N is an element that contributes to the improvement of corrosion resistance. However, as described above, when electron beam welding is performed in vacuum, N in steel is released. Therefore, even if N is released, 0.20% or more is contained so that sufficient N remains in the welded portion.
However, if the N content exceeds 0.30%, welding defects (blowholes) occur. Therefore, the proper N content is
It is 0.20 to 0.30%.

Al: 0.010 to 0.050% Al cannot contain Ti as a deoxidizing agent for the reason described below. Therefore, instead of Ti, 0.010% as a deoxidizing agent is used.
% Or more. However, if the Al content exceeds 0.050%, AlN is formed and the toughness is reduced. Therefore, Al
The content is 0.010 to 0.050%.

Ca: 0 to 0.0050%, B: 0 to 0.0030% In order to compensate for the decrease in hot workability due to the high N content, Ca and / or B may be added if necessary. But Ca
If the content of Ca exceeds 0.0050%, the amount of CaS produced increases and corrosion resistance deteriorates. Further, if the content of B exceeds 0.0030%, the grain boundary precipitation of Cr ₂₃ (C, B) ₆ is promoted and the corrosion resistance is impaired.

The duplex stainless steel of the present invention comprises the above components, the balance Fe and impurities. P, S, T in impurities
i, Nb and V are regulated as follows.

P: 0.035% or less P is an impurity that increases the susceptibility to high temperature welding cracking of steel.

S: 0.005% or less S is an impurity which deteriorates hot workability and corrosion resistance. Therefore, it is 0.005% or less, and the lower the better. 0.002% or less is more preferable.

Ti: 0.05% or less Ti originally has a function as a deoxidizing agent, but forms N and TiN as described above and reduces the toughness of the welded portion.
Its content is 0.05% or less. In order to more reliably suppress the formation of TiN, the Ti content is preferably 0.01% or less.

Nb: 0.1% or less Nb also forms a nitride and reduces the toughness of the welded portion.
Its content is 0.1% or less. 0.05 is more desirable
% Or less.

V: 0.5% or less If the content of V exceeds 0.5%, the formation of σ phase is promoted and the hot workability of steel deteriorates. Therefore, V should be suppressed to 0.5% or less. Since V has an effect of improving corrosion resistance, V may be contained in order to obtain the effect in the range of 0.5% or less.

Duplex stainless steel has a ferrite content of about 50% at room temperature and the rest is austenite. However, since it has a ferrite single phase structure at high temperature, when electron beam welding is performed, the welded part once becomes a ferrite single phase, and after solidification, a structure in which austenite is slightly precipitated and a large amount of ferrite is formed. Especially in electron beam welding, the heat input is small, so the cooling effect of the base metal is large,
The solidification rate of the weld is fast. Therefore, the welded part has a rapidly solidified structure in which the structure at high temperature is retained, and the amount of ferrite increases. A welded portion containing excessive ferrite not only has a low toughness but also a low corrosion resistance. In particular, the increase in ferrite increases the risk of delayed fracture due to hydrogen. In order to prevent the deterioration of properties due to such microstructural changes, the amount of ferrite in the weld is 80% of the weld.
The following is acceptable.

In the duplex stainless steel of the present invention, when electron beam welding is performed, the amount of ferrite in the weld is 80% of that of the weld.
% Or less. Therefore, the radioactive substance storage container manufactured by electron beam welding of this duplex stainless steel has excellent toughness and corrosion resistance at the welded portion.

The second invention relates to a method of manufacturing a radioactive substance storage container using the duplex stainless steel of the present invention.

In electron beam welding, heat input management is important. In particular, since the duplex stainless steel of the present invention has a high N content, N is released during welding. If it is not released to the outside of the welded part and is trapped in the welded part, defects such as blow holes tend to remain.

In order to prevent the above defects from occurring, the welding energy per unit area at the time of electron beam welding is set to 7.7 to 1
It is necessary to set it to 8.0 kJ / cm ² . If the welding energy is less than 7.7 kJ / cm ² , surface defects occur on the outer surface of the weld due to insufficient heat input. On the other hand, if it exceeds 18.0 kJ / cm ² , the heat input is too large and the amount of nitrogen diffused into the welded portion (molten pool) increases, and a large amount of bubbles are formed, causing internal defects.

When the duplex stainless steel of the present invention is welded by electron beam welding, a radioactive substance storage container having extremely high dimensional accuracy can be obtained. The weld of the structure has no welding defects and has excellent corrosion resistance. In addition, there is no delayed fracture due to hydrogen, and the impact value is high. These properties are essential properties for radioactive material storage containers.

[0042]

[Examples] 50 kg of duplex stainless steel whose composition was adjusted as shown in Table 1 was melted in a high frequency electric furnace, forged and hot rolled, then heated and held at 1080 ° C for 30 minutes, water cooled to a thickness. 20 mm steel plate was manufactured. Using this steel sheet as the base material, 1.33 × 10 ^-2
Electron beam welding was performed in a container whose vacuum degree was increased to Pa (1 × 10 ⁻⁴ Torr). When performing welding, the welding energy input was changed by changing the welding current, accelerating voltage, welding speed, etc. to determine the proper welding conditions.

For the test material after welding, defect observation, Charpy impact test, delayed fracture embrittlement test and pitting corrosion potential were measured, and the ferrite ratio of the weld metal was calculated.

In the defect observation, the appearance of the welding defect was visually observed and the inside of the welded portion was observed by the X-ray transmission method. Moreover, the presence or absence of internal defects such as voids was investigated by observing the cross section of the welded portion with an optical microscope.

In the Charpy impact test, a V-notch impact test piece defined by JIS Z 2202 was prepared so that only the welded portion of the test material became the notch portion, and in accordance with JIS Z 2242, it was −50.
The impact value (absorbed energy per 1 cm ² ) at ℃ was measured.

In the delayed fracture embrittlement test, a test piece including a welded part and having a parallel portion of 3φ × 20 mm and a total length of 70 mm was prepared, and this test piece was dissolved in 5% sulfuric acid in a solution of 1.4 g of thiourea per liter. Then, a stress of 600 MPa was applied to generate hydrogen by cathodic electrolysis. At this time, the temperature of the solution was 35 ° C. and the current density was 0.1 mA / cm ^2, and after 300 hours, it was examined whether or not the material was broken.

In measuring the pitting corrosion potential, a test piece containing the weld, HAZ and weld metal was sampled from the test material, and the test piece and SCE (standard electrode) were used as electrodes to prepare artificial seawater (85 ° C). The pitting potential was measured in a commercially available artificial seawater chemical for metal corrosion test).

The ferrite ratio of the weld metal was calculated by etching the cross section of the weld metal with oxalic acid electrolysis and KOH electrolysis and performing image analysis using a 500 times micrograph.

Table 1 shows the chemical composition of the test material and the welding energy when electron beam welding is performed.

[0050]

[Table 1]

Table 2 is a table summarizing the results of the above tests. In the evaluation of welding defects in the table, the case where there is no defect is indicated by ◯, and the case where a defect occurs is indicated by x. In addition, the evaluation of delayed fracture is indicated by ◯ when there is no fracture and by X when there is fracture.

[0052]

[Table 2]

As can be seen from Tables 1 and 2, the sample materials (No. 1 to 5) obtained by electron beam welding the duplex stainless steel of the present invention under the conditions specified in the present invention are all ferrite ratios in the weld metal. Is 80% or less, there are no welding defects,
The impact value is also large, and delayed fracture brittleness is not observed. Moreover, these have a high pitting corrosion generation potential and are also excellent in corrosion resistance.

On the other hand, the test material with a high S content (No. 6)
The pitting corrosion potential is low and the corrosion resistance is poor. It is considered that this is because the sulfide formed in the weld metal promoted the occurrence of pitting corrosion. Similarly, the test material with a low Cr content (No. 7) also has poor corrosion resistance.

The test material (No. 8) having a high Ti content has not only poor corrosion resistance but also low impact value. It is considered that this is because Ti has the effect of fixing N, which has an effect on corrosion resistance, and the formation of TiN has deteriorated toughness and promoted the occurrence of pitting corrosion due to dissolution in the vicinity of inclusions.

The test material (No. 9) having a low Al content has a low pitting corrosion generation potential and poor corrosion resistance. In addition to this, since the ferrite ratio in the weld metal exceeds 80%, the delayed fracture embrittlement resistance also deteriorated. Also, a test material with a high Al content (No. 10)
The shock value is low. It is considered that this is because Al combined with N to form AlN.

The test material (No. 11) having a low N content showed a decrease in impact value and a decrease in pitting corrosion generation potential, mechanical properties,
Not enough corrosion resistance. Furthermore, the ferrite ratio in the weld metal exceeded 80%, and the delayed fracture embrittlement resistance also deteriorated.
Further, in the test material (No. 12) having an excessively high N content, welding defects occurred.

N based on the duplex stainless steel of the present invention
o.13 and 14 are examples of improper heat input for electron beam welding. Weld defects occurred in these.

[0059]

The duplex stainless steel of the present invention is excellent in electron beam weldability and has good corrosion resistance and mechanical properties in the welded portion even after welding. Defects will not occur if electron beam welding is performed by applying appropriate welding energy to such duplex stainless steel. Therefore, the duplex stainless steel of the present invention is extremely suitable as a material for a radioactive substance storage container assembled by electron beam welding.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G21F 5/00 B23K 103: 04 // B23K 101: 12 G21F 5/00 K 103: 04 G21C 19/06 S (72) Inventor Takashige Shige 2-1-1, Niihama, Arai-cho, Takasago-shi, Hyogo San Takahashi Industry Co., Ltd. Takasago Research Institute (72) Inventor Iwaji Abe 1-1-1, Wadasaki-cho, Hyogo-ku, Kobe-shi, Hyogo Mitsubishi Heavy Industries Ltd. Company Kobe Shipyard (72) Inventor Haruhiko Kajimura 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Shinji Tsuge 4-53, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Takaaki Matsuda 4-53-3 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4E066 CB0 2

Claims (2)

[Claims] 1. C: 0.005 to 0.030% by mass%, Si: 0.05
~ 0.75%, Mn: 0.20-1.00%, Ni: 5.5-7.5%, Cr: 2
4.0 to 26.0%, Mo: 2.5 to 3.5%, Cu: 0.2 to 0.8%, W:
0.1 to 0.5%, N: 0.20 to 0.30%, Al: 0.010 to 0.050%,
Ca: 0 to 0.0050%, B: 0 to 0.0030%, balance Fe and impurities, P in impurities is 0.035% or less, S is 0.005
%, Ti is 0.05% or less, Nb is 0.1% or less, and V is 0.5% or less. A duplex stainless steel for a radioactive substance storage container having excellent electron beam weldability. ^2. A method of manufacturing a radioactive substance storage container, characterized in that the duplex stainless steel according to claim 1 is assembled by electron beam welding having a welding energy per unit area of 7.7 to 18.0 kJ / cm ² .